Multi-probe gauge for slab characterization

ABSTRACT

The present subject matter at least provides an apparatus for inspecting a slab of material including a passivation layer. The apparatus includes a frequency-domain optical-coherence tomography (OCT) probe configured to irradiate the slab of material, and detect radiation reflected from the slab of material. The apparatus also includes a spectral-analysis module configured to analyze at least an interference pattern with respect to the OCT probe to thereby determine a thickness of the slab of the material. The apparatus also includes a thin-film gauge configure to determine a thickness of the passivation layer such that the determined thickness of the slab of material may be adjusted baes on the thickness of the passivation layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.15/410,328 filed on Jan. 19, 2017, and incorporated herein by referencein its entirety. This application is a continuation-in-part with respectto U.S. application Ser. No. 16/277,582, filed on Feb. 15, 2019, whichis a continuation application of U.S. application Ser. No. 15/914,445filed on Mar. 7, 2018, both of which are incorporated by reference intheir entireties.

TECHNICAL FIELD

The embodiments discussed in this disclosure are related to amulti-probe gauge for slab characterization.

BACKGROUND

Frequency domain based optical coherence tomography (FD-OCT) probes havebeen used for characterization of properties of slabs of materials suchas slab-thickness, slab structure and topography of the slabs. Forultra-thin slabs, industrially used measurement metrology is based onlow-coherence interferometry. Such techniques are based upon an analysisof reflected infrared photon flux arriving from slab surfaces(layer-interfaces). The slab-thickness and characterization of the slablayer structure is obtained by analysis of an interferogram resultingfrom the FD-OCT probes.

A parameter that is often considered during metrology is a throughputmeasured in ‘number of slabs’ a tool can measure in a unit of time(usually expressed in slabs per hour). However, the throughput of thetool-measuring slabs is limited in the case of FD-OCT probes at-leastdue to limitations within the existing slab-handling anddata-acquisition techniques. More specifically, data-acquisition speedis limited by the speed at which spectra can be acquired, which is inturn limited at least owing to independent operation of the plurality ofprobes positioned around the slab of material. In addition,conventional-mechanisms based on FD-OCT probes fail to take into accountinfluence of vibration of the slab of material during the process ofslab-characterization.

The subject matter claimed in this disclosure is not limited toembodiments that solve any disadvantages or that operate only inenvironments such as those described above. Rather, this background isonly provided to illustrate one example technology area where someembodiments described in this disclosure may be practiced.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified format that are further described in the detailed descriptionof the present disclosure. This summary is neither intended to identifykey or essential inventive concepts of the disclosure, nor is itintended for determining the scope of the invention or disclosure.

The present subject matter at least provides an apparatus for inspectinga slab of material including a passivation layer. The apparatus includesa frequency-domain optical-coherence tomography (OCT) probe configuredto irradiate the slab of material, and detect radiation reflected fromthe slab of material. The apparatus also includes a spectral-analysismodule configured to analyze at least an interference pattern withrespect to the OCT probe to thereby determine a thickness of the slab ofthe material. The apparatus also includes a thin-film gauge configure todetermine a thickness of the passivation layer such that the determinedthickness of the slab of material may be adjusted baes on the thicknessof the passivation layer.

To further clarify advantages and features of the invention claimedherein, example descriptions and embodiments are rendered by referenceto specific embodiments thereof, which is illustrated in the appendeddrawings. It is appreciated that these drawings depict only exampleembodiments of the invention and are therefore not to be consideredlimiting of its scope. The disclosure will be described and explainedwith additional specificity and detail with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates an example representation of system within a probeoperating as a frequency domain optical coherence tomography (FD-OCT)probe;

FIG. 2 illustrates a second example representation of a system withinthe probe operating as the FD-OCT probe;

FIG. 3 illustrates a third example representation of a system within theprobe operating as the FD-OCT probe;

FIG. 4 illustrates an example beam-assembly that may be implementedwithin the systems of FIG. 1-3;

FIG. 5 illustrates a fourth example representation of a system withinthe probe acting as the FD-OCT probe;

FIG. 6 illustrates a system comprising synchronized multiple FD-OCTprobes for simultaneous measurement of properties of a slab of material;

FIG. 7 illustrates a symmetric arrangement of two FD-OCT probes formeasurement of the transparent and nontransparent slabs of materials forreduction of the noise caused by vertical mechanical vibration of a slabof material;

FIG. 8 illustrates a type of asymmetric arrangement of two FD-OCT probesresulting from the relative motion of the slab of material with respectto probes due to vibration during the first portion of the vibrationwith respect to the slab of material;

FIG. 9 illustrates another type of asymmetric arrangement of two FD-OCTprobes with respect to the slab of material, resulting from motion ofthe slab of material due to vibration during the second portion ofvibration cycle shown in FIG. 8;

FIG. 10 illustrates occurrence of error during a slab-thicknessmeasurement in case of the arrangement of the FD-OCT probes as depictedin FIG. 8 and FIG. 9 resulting from the vibratory-motion of the slab ofmaterial with respect to the FD-OCT probes;

FIG. 11 illustrates synchronized FD-OCT probes utilizing aspectrometer-detector combination for measurement of properties of aslab of material;

FIG. 12 illustrates operation of a spectrometer-detector combination asconnected to the synchronized FD-OCT probes;

FIG. 13 illustrates synchronized FD-OCT probes utilizing aspectrometer-detector combination through an optical-switch formeasurement of properties of the slab of material;

FIG. 14 illustrates a flowchart of an example method of measurement ofproperties of the slab of material through synchronized FD-OCT probes;

FIG. 15A illustrates an arrangement of two FD-OCT probes for measurementof a slab of material;

FIG. 15B illustrates another arrangement of two FD-OCT probes formeasurement of a slab of material;

FIG. 16A illustrates an arrangement an FD-OCT probe for measurement of aslab of material that includes multiple layers;

FIG. 16B illustrates a simulated measurement of a slab of material thatincludes multiple layers as obtained by an FD-OCT probe;

FIG. 17A illustrates an arrangement an FD-OCT probe for measurement of aslab of material that includes multiple layers;

FIG. 17B illustrates a simulated measurement of a slab of material thatincludes multiple layers as obtained by an FD-OCT probe;

FIGS. 18A-18B illustrates a flowchart of an example process of measuringa topography of a slab of material;

FIG. 19 illustrates an example system for inspecting a slab of material,including multiple frequency domain optical coherence tomography(FD-OCT) probes and a thin-film gauge;

FIG. 20 illustrates a flowchart of an example process of measuringproperties of a slab of material that includes multiple layers;

FIG. 21 illustrates a flowchart of another example process of measuringproperties of a slab of material that includes multiple layers;

FIG. 22 illustrates a computing device configured to facilitatemeasurement of properties of a slab of material through synchronizedFD-OCT probes.

The elements in the drawings are illustrated for simplicity and may nothave been necessarily been drawn to scale. Furthermore, in terms of theconstruction of the device, one or more components of the device mayhave been represented in the drawings by conventional symbols, and thedrawings may show only those specific details that are pertinent tounderstanding the embodiments of the present disclosure so as not toobscure the drawings with details that will be readily apparent to thoseof ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the present disclosure is thereby intended, suchalterations and further modifications in the illustrated system, andsuch further applications of the principles of the present disclosure asillustrated therein being contemplated as would normally occur to oneskilled in the art to which the present disclosure relates.

The foregoing general description and the following detailed descriptionare explanatory of the present disclosure and are not intended to berestrictive thereof.

Reference throughout this specification to “an aspect”, “another aspect”or similar language means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, appearancesof the phrase “in an embodiment”, “in another embodiment” and similarlanguage throughout this specification may, but do not necessarily, allrefer to the same embodiment.

The terms “comprises”, “comprising”, or any other variations thereof,are intended to cover a non-exclusive inclusion, such that a process ormethod that comprises a list of steps does not include only those stepsbut may include other steps not expressly listed or inherent to suchprocess or method. Similarly, one or more devices or sub-systems orelements or structures or components proceeded by “comprises . . . a”does not, without more constraints, preclude the existence of otherdevices or other sub-systems or other elements or other structures orother components or additional devices or additional sub-systems oradditional elements or additional structures or additional components.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this present disclosure belongs. The system, methods,and examples provided herein are illustrative only and not intended tobe limiting.

Embodiments of the present disclosure will be described below in detailwith reference to the accompanying drawings.

FIG. 1 illustrates an example representation of a system 100 orarrangement within a probe operating as a frequency domain opticalcoherence tomography (FD-OCT) probe for measuring one or more propertiesof a slab of material 102. In some embodiments, the slab of material 102may be any suitable piece of material that may have a relatively flatsurface. For example, the slab of material 102 may be a silicon wafer, asemiconductor wafer, a glass plate or glass sheet (e.g., glass sheetsused for wafer carriers, windows, flat panel displays, etc.), apolyester sheet (e.g., used for flexible panel displays), a polyimidesheet (e.g., used for flexible panel displays), sheet metal, a sandwichof various materials, such as those discussed above, or any otherrelatively flat element in which measuring properties of the elementsuch as thickness or topography may be performed. A description andexample of the system 100 can also be found at-least with respect toembodiments defined under FIG. 1 of the U.S. patent application Ser. No.15/410,328, filed Jan. 19, 2017, the complete-disclosure of which isincorporated herein by reference in its entirety.

The system 100 configured to inspect the slab of material 102 isarranged within the FD-OCT probe and accordingly denotes an internalassembly. In general, the system 100 may be configured to inspect theslab of material 102 in order to determine one or more properties of theslab of material 102 such as, for example, a thickness 106 of the slabof material 102 and a topography of a front surface 104 and/or a backsurface 105 of the slab of material 102. To perform the inspection, thesystem 100 may include single mode optical fibers 108, 110, 112, and114, a broadband light source 116, a beam forming assembly 118, adirectional element 126, and an etalon filter 120 and a spectrometer 122both controlled by a computer 124.

The broadband light source 116 may be configured to emit light over theoptical fiber 108. The directional element 126 may be configured toreceive the light from the broadband light source 116 over the opticalfiber 108 and direct the light to the beam assembly 118 over the opticalfiber 110. The beam assembly 118 may be configured to receive the lightover the optical fiber 110 and direct the light toward the slab ofmaterial 102, i.e. irradiate the slab of material 102. The beam assembly118 may be further configured to receive the light reflected from theirradiated slab of material 102 and direct the light back to thedirectional element 126 over the optical fiber 110. The etalon filter120, as controlled by the computer 124, may be configured to receive thelight over the optical fiber 112 after the light has been reflected fromthe slab of material 102, filter the light, and direct the light overthe optical fiber 114. The spectrometer 122, as controlled by thecomputer 124, may be configured to receive the light over the opticalfiber 114, after the light has been filtered by the etalon filter 120and after the light has been reflected from the slab of material 102,and spectrally analyze the light. More specifically, the spectrometer122 may be coupled to one-dimensional array-detector (e.g. a chargecoupled device) to capture electric-charge corresponding to aninterference pattern (e.g., an interferogram) formed due to interferenceamong the diffracted-frequencies due to dispersion of reference andreflected light within the spectrometer 122. The interferogram asobtained from the array-detector is used for spectral-analysis of thereflected-light from the slab of material 102. In some examples, the onedimensional array detector may be in-built within the spectrometer 122.

The spectral analysis of the light may include determining topography ofthe front surface 104 and/or the back surface 105 of the slab ofmaterial 102 and/or determining the thickness 106 of the slab ofmaterial 102 including determining a thickness of one or more layers ofthe slab of material. For such purposes, the computer 124 may beelectrically coupled to the etalon filter 120 and to the spectrometer122, and operates as a spectral-analysis module. The computer 124 may beconfigured to determine the thickness 106 of the slab of material 102,the thickness of the layers of the slab of material, and/or determininga topography of the front-surface 104 the back surface 105 of the slabof material 102.

The computing-device 124 executes instructions to perform operationswith respect to the spectrometer 122 in order to determine a topographyof the front surface 104 and/or the back surface 105 of the slab ofmaterial 102 and/or determine the thickness 106 of the slab of material102. In an example, the instructions to measure the thickness 106 of theslab of material 102 correspond to the following sequential-procedure asillustrated with respect to FIG. 1 in U.S. patent application Ser. No.15/410,328:

-   -   1. Measurement of the reference spectrum (as shown in FIG. 9A of        U.S. patent application Ser. No. 15/410,328) of the broadband        light source 116    -   2. Measurement the signal spectrum (as shown in FIG. 9B of U.S.        patent application Ser. No. 15/410,328) of the light reflected        from the slab of material 102 having a known refractive index n,        and passing through the etalon filter 120 having a known        thickness which is known to be slightly larger than the        thickness 106 of the measured slab of material 102.    -   3. Calculating a normalized spectrum (as shown in FIG. 9C of        U.S. patent application Ser. No. 15/410,328) by dividing the        signal spectrum by the reference spectrum.    -   4. Calculating the frequency Ω of observed oscillations in the        normalized spectrum.    -   5. Calculating the thickness 106 of the slab of material 102        using Equation 34 of U.S. patent application Ser. No.        15/410,328.

The topography of the front surface 104 of the slab of material 102 maybe determined by placing the slab of material 102 on an XY motion stageperpendicular to light beam impinging front surface 104, wherein thefront surface 104 is parallel to plane of motion of XY stage, and bycollecting a data-set comprising the data set on a large number Mcomprising the x_(j) and y_(j) coordinates of the point where the beamis impinging the front surface 104 of the slab of material 102 and thedistance between the a stationary lens (shown in FIG. 4 as 404) of thebeam-forming assembly 118 and the front surface 104 of the slab ofmaterial 102 z_(j), where j=1 . . . M. The set of points (x_(j), y_(j),z_(j)) can then be used to construct a three dimensional map of thefront surface 104 of the slab of material 102. A similar procedure maybe performed to determine the topography of the back surface 105 of theslab of material 102.

FIG. 2 illustrates another example system 200 or arrangement within theone probe operating as the FD-OCT probe for inspecting a slab ofmaterial. A description and example of the system 200 may also be foundat least with respect to embodiment defined under FIG. 2 of theaforesaid U.S. patent application Ser. No. 15/410,328.

The system 200, in addition to components referred with the system 100,further includes a second directional element 213, an etalon filter 220,and a single mode optical fiber 215. The second directional element 213may be configured to receive the light from the directional element 126over the optical fiber 112 and direct the light to the etalon filter 220over the optical fiber 215. The etalon filter 220 may be configuredsimilarly to the etalon filter 120 of FIG. 1, except that the etalonfilter 220 may be configured to receive the light from the seconddirectional element 213 over the optical fiber 215 after the light hasbeen reflected from the irradiated slab of material 102 and direct thelight back to the second directional element 213 over the optical fiber215. The spectrometer 122 of the system 200 may then be configured toreceive the light from the second directional element 213 over theoptical fiber 114.

FIG. 3 illustrates a third example representation of a system 300,within the probe operating as the FD-OCT probe, for inspecting the slabof material, arranged in accordance with at least some embodimentsdescribed in this disclosure. An example description of the system 300may also be found at least with respect to embodiment defined under FIG.3 of the aforesaid U.S. patent application Ser. No. 15/410,328.

The system 300, in addition to elements in common with the system 100,may include a single mode optical fiber 317 and an etalon filter 320.The etalon filter 320 may be configured similarly to the etalon filter120 of FIG. 1, except that the etalon filter 320 may be configured toreceive the light over the broadband light source 116 over the opticalfiber 317 before the light is directed toward the slab of material 102and then, after filtering the light, direct the light over the opticalfiber 108 to the directional element 126. Then, after the light has beenreflected from the slab of material 102, the spectrometer 122 may beconfigured to receive the light from the directional element 126 overthe optical fiber 112.

FIG. 4 illustrates an example beam assembly 400 containing a referenceplane enabling absolute distance metrology, arranged in accordance withat least some embodiments described in this disclosure and implementablewithin the systems of FIG. 1-3 of the present subject matter. An exampledescription of the assembly 400 can also be found at least with-respectto a beam-assembly 500, as defined under FIG. 5 within the aforesaidU.S. patent application Ser. No. 15/410,328.

The beam assembly 400 may be employed as the beam assembly 118 in thesystems 100, 200 and 300 of FIG. 1 through FIG. 3. The beam assembly 400may include lenses 402 and 404. The beam assembly 400 may alsooptionally include a beam splitter 406 and a reflector 408. The lens 402may be configured to receiving the light over the optical fiber 110 andcollimate and direct the light toward the beam splitter 406. The beamsplitter 406 may be configured to split the light from the lens 402 intofirst and second portions, direct the first portion of the light towardthe lens 404, and direct the second portion of the light onto areflector 408. The lens 404 may be configured to receive the firstportion of the light from the beam splitter 406, direct the firstportion of the light toward the slab of material 102, and direct thefirst portion of the light after being reflected from the slab ofmaterial back toward the beam splitter 406. Further, the reflector 408may be configured to receive the second portion of the light from thebeam splitter 406 and reflect the second portion of the light backtoward the beam splitter 406. The beam splitter 406 may be furtherconfigured to combine the first portion of the light after beingreflected from the slab of material 102 and the second portion of thelight after being reflected from the reflector 408, and then direct thecombined light toward the lens 402. Finally, the lens 402 may beconfigured to receive the combined light and direct the combined lightover the optical fiber 110.

The beam assembly 400 may be employed to gauge the optical pathdifference (OPD) between the first portion of the light and the secondportion of the light, which can be used to measure the distance betweenthe front surface 104 of the slab of material 102 and the lens 404. Ashas been depicted with respect to the description in FIG. 1, distancebetween the front surface 104 of the slab of material 102 and the lens404 assists in determination of topography of front and back surfaces ofthe slab of material 102.

FIG. 5 illustrates a fourth example system 500 within the FD-OCT probefor inspecting a slab of material, in accordance with at least someembodiments described in this disclosure. An example description of saidsystem 500 can also be found at least with-respect to the system asdefined under FIG. 4 of the aforesaid U.S. patent application Ser. No.15/410,328.

Although the beam splitter 406 and the reflector 408 may be beneficialin some embodiments of the beam assembly 400 as noted above with respectof FIG. 4 of the present subject matter, the present FIG. 5 omits thebeam splitter 406 and the reflector 408. For example, in addition toelements in common with the system 100, the system 500 may include afirst beam assembly 518 a and a second beam assembly 518. Since thelight only passes through the beam assemblies 518 a and 518 b in asingle direction in the system 500, the beam splitter 406 and thereflector 408 (otherwise present in FIG. 4 of the present subjectmatter) may be omitted.

More specifically, the beam assembly 518 a may be similar to the beamassembly 118 of FIG. 1 except that the beam-assembly 518 a is notconfigured to receive the light reflected back from the slab of material102. Instead, the light directed from the beam assembly 518 a istransmitted through the slab of material 102 toward the second beamassembly 518 b. The second beam assembly 518 b may be configured toreceive the light transmitted through the slab of material 102 anddirect the light to the etalon filter 120 over the optical fiber 112.The etalon filter 120 may then be configured to receive the light fromthe second beam assembly 518 b over the optical fiber 112 after thelight has been transmitted through the slab of material 102.Accordingly, the characterization of the slab of material 102 in thepresent FIG. 5 is based on light transmitted through the slab ofmaterial 102.

FIG. 6 illustrates a system 600 comprising multiple synchronized FD-OCTprobes for simultaneous measurement of properties of a slab of material.More specifically, FIG. 6 describes multiple probes 602 (1) . . . 602(n)for acquiring data synchronously for measurement of thickness andtopography of the slabs of material. The assembly of each of said FD-OCTprobe 602 (1 to n) within the system 600 corresponds to the systems 100,200, 300 and 500 as have been referred in the previous figures, whereineach of the probe 602 is an assembly of single mode optical fibers 108,110, 112, 215, 317 and 114, the broadband light source 116, the beamforming assembly 118, the directional elements 126, 213, the etalonfilter 120, 320 and the spectrometer 122. Further, all of the FD-OCTprobes 602 in the system 600 measure a common slab of material 102 andconnected to the common spectral-analysis module or the computing system124. For sake of brevity, only two FD-OCT probes 602(1) and 602(n) havebeen shown as a part of the system 600.

Within the present system 600, the upper FD-OCT probe 602(1) issynchronized with lower FD-OCT probe 602(n) by means of a centralizedactuation mechanism 611 or a common electrical-cable 611 whose one endis connected to all the FD-OCT probes. The other end of suchelectrical-cable 611 is connected to a triggering source (e.g. amicrocontroller) that sends pulses to actuate each of said FD-OCT probesat the same time such that the FD-OCT probes simultaneously detect thereflected radiation from the slab of material 102 and thereby enable asimultaneous measurement of distances from the slab of material 102 toeach of the FD-OCT probes. In the present disclosure, reference tooperations happening “simultaneously” or “at the same time” or being“synchronized” allows for margins of error in the simultaneous natureof, that may be less than 1%, 5%, or 10% depending on variousimplementation or materials constraints. Accordingly, upon havingreceived a trigger through the electrical cable 611, each of the probeout of the plurality of FD-OCT probes 602 (1 to n) operatessimultaneously. In other example, the actuation mechanism 611 may beelectro-mechanical, opto-electrical or opto-mechanical in nature.Largely, the simultaneous operation of the each of the probe 602 may bedefined as a sequential-operation of:

a) irradiating the slab of material at a particular location;

b) detecting radiation reflected from the slab of material ortransmitted there-through;

Further, as may be understood with respect to FIG. 6, each of pluralityof the probes 602 (1 to n), inter alia, includes a combination ofspectrometer 122 and uni-dimensional array detector 124. In other words,each of the probes 602 includes a respective spectrometer-array detectorcombination, wherein the spectrometer and array detector may either beintegrated or separate from each other.

FIG. 7 illustrates a symmetric arrangement of two FD-OCT probes formeasurement of the transparent and nontransparent slabs of materials forreduction of the noise caused by vertical mechanical vibration of a slabof material. More specifically, FIG. 7 illustrates a system 700 ofFD-OCT probes for the purposes of reducing the influence of mechanicalvibrations of the slab of material 102 during the course of determiningthickness of the slab of material 102, which may be transparent,semi-transparent, or non-transparent in nature. As shown in FIG. 7, thesystem 700 comprises a pair of probes 602(1) and 602(2). ‘D’ representsa distance between the two probes 602(1) and 602(2) that may bepre-determined through any known means. Further, the distances H1 andH2, respectively depict the distance of the slab of material 102 fromthe first probe 602 (1) and the second probe 602(2). Said distances aredetermined based on the spectral analysis performed with respect to eachof the probe 602 (1) and 602 (2), as illustrated through the procedureillustrated in FIG. 1. Thereafter, the thickness ‘t’ of the slab ofmaterial 102 is determined based on following criteria (as illustratedin U.S. Pat. No. 7,116,429):

t=D−H1−H2

The simultaneous calculation of H1 and H2 owing to a synchronizedoperation of the probe 602(1) and 602(2) coupled with a symmetricarrangement of probes 602 (1) and 602(2) with respect to the slab ofmaterial 102 causes a precise determination of distances H1 and H2 andthereby a precise determination of the thickness ‘t’. Such measurementovercomes the influence of mechanical vibrations of the slab of material102 during the course of determining thickness of the slab of material102.

FIG. 8 illustrates a type of asymmetric arrangement of two FD-OCT probesresulting from the relative motion of the slab of material with respectto probes due to vibration during the first portion of the vibrationwith respect to the slab of material.

On the other hand, FIG. 9 illustrates another type of asymmetricarrangement of two FD-OCT probes with respect to the slab of material,resulting from motion of the slab of material due to vibration duringthe second portion of vibration cycle shown in FIG. 8.

Both FIG. 8 and FIG. 9 illustrate a non-synchronized arrangement of theprobes 602(1) and 602(2), as a result of which both probes determine thedistance from the slab of material 102 at different instants of time,say at t1 and t2, respectively. As illustrated in FIG. 8, at time ‘t1’,the distance between probe 602(1) and the slab of material 102 isobserved to be H1A. As illustrated in FIG. 9, at time ‘t2’, the distancebetween probe 602(2) and the slab of material 102 is observed to be H2A.The asymmetric arrangement of two FD-OCT probes as depicted in FIG. 8and FIG. 9 result from the relative motion of the slab of material withrespect to probes due to vibration upwards and downwards, respectively.Such upward and downward vibration may also be referred as first andsecond portion of the vibration or vibratory-motion. Based on saidreadings, the thickness ‘tA’ gets determined as follows:

tA=D−H1A−H2A

FIG. 10 illustrates occurrence of error during a slab-thicknessmeasurement in case of the arrangement of the FD-OCT probes as depictedin FIG. 8 and FIG. 9 resulting from the vibratory-motion of the slab ofmaterial with respect to the FD-OCT probes. Clearly, as indicated inFIG. 10, thickness ‘tA’ is substantially greater than the actualthickness ‘t’ of the slab of material 102 and gets determined as asubstantially high thickness of the slab of material 102 owing to acontinuous-vibration of the slab of material 102 with respect to theprobes along y axis and non-synchronized operation of the probes 602(1)and 602(2). Accordingly, as may be understood, the synchronizedoperation of the probes 602(1) and 602(2) is able to determine theprecise thickness ‘t’ despite the vibration of the slab of material 102by virtue of measurement of the distances H1 and H2 as the same instantof time.

FIG. 11 illustrates synchronized FD-OCT probes utilizing aspectrometer-detector combination for measurement of properties of aslab of material. More particularly, the system 1100 depicted in FIG. 11has synchronized FD-OCT probes 602 utilizing a centralized or solespectrometer-detector combination for measurement of thickness andtopography of slabs of material. More specifically, neither of theFD-OCT probes 602 in the system 1100 comprises an in-built spectrometerand detector combination. Instead, each of the probes 602 is ratheraligned to a centralized spectrometer 1102 and detector 1104, therebyincurring significantly lower manufacturing costs when compared with thesystem 600. Further, the detector 1104 within the system is a 2dimensional (2D) array detector, e.g., a charge coupled device havingmultiples rows of photo-sensitive elements.

FIG. 12 illustrates operation of a spectrometer-detector combination asconnected to the synchronized FD-OCT probes. The spectrometer 1102 asillustrated in FIG. 12 is a dispersive-element based spectrometer. Asshown in the figure, the light 1208, 1210 arriving from the slab ofmaterial 102 is directed to a same entrance slit 1202 of the dispersivespectrometer 1102 and accordingly positioned in separate locations alongthe entrance slit 1202, or in other words at different locations withinan object-plane of spectrometer 1102. Upon having undergone diffractionfrom a dispersive element i.e. grating 1204 of the spectrometer 1102, aplurality of dispersed images of the slit 1202 are projected on thetwo-dimensional array detector 1104.

The light entering through the different points on the same slit 1202form spectral lines across the detector 1104, such that each row (i.e. auni-dimensional array) of light sensitive elements corresponds to aparticular spectral line or spectrum out of the plurality of spectrumscaptured by the detector 1102. More specifically, each light originatingfrom the slab of material 102 undergoes dispersion to produce aplurality of diffracted wavelengths that interfere with one another toproduce a corresponding interferogram with respect to a particularlight. Overall, a plurality of interferograms are captured as aplurality of spectral lines through the array detector 1102. The voltagesignal corresponding to various spectral lines (i.e. each row of thearray-detector) is multiplexed and thereafter digitized forserial-transmission through a cable to computer 124 (i.e. spectralanalysis module) that further analyzes spectra in accordance with the‘procedure’ as described within the description of FIG. 1. Accordingly,each spectral-line corresponding to each light received from probe 602(1 to n) is analyzed separately through spectral analysis module.

FIG. 13 illustrates synchronized FD-OCT probes utilizing aspectrometer-detector combination through an optical-switch formeasurement of properties of the slab of material. A system 1300 asshown in FIG. 13 comprises synchronized FD-OCT probes utilizing thecentralized spectrometer and detector combination (as depicted throughFIG. 11) through an optical-switch 1302 for measurement of thickness andtopography of the slab of material. More specifically, each of theplurality of probes 602 (1 to n) are ported to the centralizedspectrometer 1102 through an optical-switch 1304. In addition, saidoptical switch 1304 may be also appropriated to link a single broadbandlight source to the multiple probes 602 (1 to n) for the purposes ofirradiating the slab of material 102. In an example, the optical switchmay correspond to either 1×N or N×1 configuration and may exemplarily bemechanical switch or Micro-electromechanical System (MEMS) switch.

FIG. 14 illustrates a flowchart of an example-method of measurement ofproperties (e.g. thickness and topography) of a slab of material throughmultiple FD-OCT probes 602 (1 to n).

At step 1402, the slab of material 102 to be measured in placed within achuck for initiating the measurement.

At step 1404, one or more elements within a plurality of FD-OCT probes602 (1 to n) are actuated at the same time by the centralized actuatingmechanism 611 to cause an operation thereof. Such operation of theplurality of probes 602 is achieved due to simultaneous actuation ofeach of the probe 602 by the actuation mechanism as described withrespect to FIG. 6 to FIG. 11 and comprises:

a) irradiating the slab of material at-least one location by saidplurality of probes 602;

and

b) synchronously detecting by the plurality of probes 602 the radiationreflected from the slab of material or transmitted there-through.

The synchronized-operation of the probes 602 in step 1404 furthercomprises filtering the detected radiation through an etalon-filter 120,220, 320 present within each of the FD-OCT probe 602 to cause generationof the interference pattern with respect to each FD-OCT probe 602. Anindividual operation of each of the FD-OCT probe 602 corresponds to themethod-steps as described with respect to the FIG. 10 of the aforesaidU.S. patent application Ser. No. 15/410,328. Further, the radiation orlight outputted from each of the FD-OCT probe 602 is dispersed by adispersive element (e.g. grating) within the spectrometer 122, 1102 tothereby generate a plurality of spectrums. Each of the plurality ofspectrums detected through the 2-dimensional array-detector 1104thereafter undergo a spectral-analysis through the next step 1406. Inorder to facilitate such spectral-analysis, a plurality of signals (e.g.voltage) corresponding to the detected-spectrums and generated at thedetector are multiplexed and thereafter digitized for transmission tothe computer 124. The digitized-signal received at the computer 124thereafter undergoes a digital to analog conversion through adigital-to-analog converter and de-multiplexing to recreate a pluralityof analog signals at the computer 124 for the purposes ofspectral-analysis.

At step 1406, the analog-signal(s) recreated at the computer 124 fromthe previous step 1404 are spectrally-analyzed (in accordance with theprocedure as described with respect to FIG. 1). Accordingly, the presentstep denotes a spectral-analysis of the interference pattern or spectrumcorresponding to each of the probes. The spectral analysis in turn leadsto a determination of properties of the slab of material such as a)thickness with respect to one or more locations at the slab of material;and b) topography of the slab of material.

FIG. 15A illustrates an arrangement of two FD-OCT probes 602 formeasurement of a slab of material 150 according to at least oneembodiment described in the present disclosure. In general, the probes602 irradiate the top and bottom of the slab of material 150, receiveradiation reflected by or transmitted through the slab of material 150,and spectrally analyze the received radiation to determine properties ofthe slab of material 150. The slab of material 150 may be similar to orbe an example of the slab of material 102 of FIGS. 1-4, 5-11, and 13.Additionally, the slab of material 150 may include multiple layers, forexample, a passivation layer, a metallic layer, and a silicon layer.

In some embodiments, the system 1550 may include a stand 101 configuredto support the probes 602. In some embodiments, the system 1550 mayinclude two or more probes 602. The probes may be the same as, orsubstantially similar to the probes 602 described above with regard toFIGS. 7-13.

In some embodiments the system 1550 may include an XY motion stage 170.The stand 101 may be configured to support the XY motion stage 170. TheXY motion stage 170 may be configured to move the slab of material 150relative to the probes 602 such that different portions of the slab ofmaterial 150 may be irradiated and measured by the probes 602 atdifferent times. The XY motion stage 170 may include a cutout 171through which radiation may pass. The cutout 171 may be a void in the XYmotion stage 170. Additionally or alternatively, the cutout 171 mayinclude a material that has a relatively small or well-known impact onradiation passing through the cutout 171.

In some embodiments, the system 1550 may include a wafer chuck 160. Thewafer chuck 160 may be configured to retain the slab of material 150 inposition relative to the XY motion stage 170. The wafer chuck 160 mayinclude a cutout 161. The cutout 161 may be a void in the wafer chuck160. Additionally or alternatively, the cutout 161 may include amaterial that has a relatively small or well-known impact on radiationpassing through the cutout 161. The wafer chuck 160 may include groovesconfigured to hold the slab of material 150. For example, the waferchuck 160 may include grooves that may be retained at a low air pressurerelative to the surrounding air such that a suction force retains theslab of material 150 on the wafer chuck 160.

In some embodiments, the system 1550 may include an optical metrologyunit 127. The optical metrology unit 127 may be configured to analyzesignals or light received by the probes 602. The optical metrology unit127 may be configured to perform spectral analysis of the radiationreceived by the probes 602. The optical metrology unit 127 may beconfigured to determine one or more properties of the slab of material150, for example, thickness and/or topology. The optical metrology unit127 may include one or more of spectrometers, etalons and/or lightsources.

In these or other embodiments, the slab of material 150 may include acomputer 124 that may be configured to analyze signals received by theprobes 602. The computer 124 may be configured to perform spectralanalysis of the radiation received by the probes 602. The computer 124may be configured to determine one or more properties of the slab ofmaterial 150, for example, thickness and/or topology. Additionally oralternatively the computer 124 may be configured to control the XYmotion stage 170 and/or the probes 602. For example the system 1550 maybe configured to calculate a distance from the probe 602 to a surface ofthe slab of material 150 and/or the thickness of the slab of material150, including the thickness of one or more layers of the slab ofmaterial 150, at a first location. Then, the system 1550 may beconfigured to move the slab of material 150 to a second location andcalculate a distance from the probe 602 to a surface of the slab ofmaterial 150 and/or the thickness of the slab of material, including thethickness of one or more layers of the slab of material, at the secondlocation. The system 1550 may be configured to scan multiple locationsof the slab of material 150 and generate a topography of the slab ofmaterial or a map of the thickness of the slab of material, includingthe thickness of one or more layers of the slab of material.

The top probe 602(1) may be connected to the optical metrology unit 127through a single-mode optical fiber 107. The bottom probe 602(2) may beconnected to the optical metrology unit 127 through a single-modeoptical fiber 109. The optical metrology unit 127 may be connected tothe computer 124 through an electrical cable 111. And, the computer 124may be connected to the XY motion stage 170 through an electrical cable113.

Modifications, additions, or omissions may be made to the system 1550without departing from the scope of the present disclosure. For examplethe system 1550 may include any number of .probes 602. As anotherexample, the optical metrology unit 127 may be included in or part ofthe computer 124.

FIG. 15B illustrates another arrangement of two FU-OCT probes 602 formeasurement of a slab of material 150 according to at least oneembodiment described in the present disclosure. The system 1560 of FIG.15B may be the same as or substantially similar to the system 1550 ofFIG. 15A with the exception of pins 130.

In some embodiments, the system 1560 may include the pins 130. The pins130 include three or more pins configured to position the slab ofmaterial 150 relative to the wafer chuck 160. The pins 130 may beattached to or part of the wafer chuck 160. The pins 130 may retain theslab of material 150 relative to the wafer chuck 160 as the XY motionstage 170 moves the wafer chuck 160. Additionally or alternatively, thepins 130 may retain the slab of material 150 relative to the wafer chuck160 even if the stand 101 is moved, shaken and/or vibrated. The pins 130may be useful in measuring a bow or warp of the slab of material 150.

Modifications, additions, or omissions may be made to the system 1560without departing from the scope of the present disclosure. For examplethe system 1560 may include any number of .probes 602 and/or pins 130.

FIG. 16A illustrates an arrangement an FD-OCT probe 602(1) formeasurement of a slab of material 150 that includes multiple layersaccording to at least one embodiment described in the presentdisclosure. The probe 602(1) may be the same as or substantially similarto the probes 602 described above with regard to FIGS. 7-13 and 15. Ingeneral, the probe 602(1) may irradiate the slab of material 150 withray 900 and ray 901, the ray 900 and the ray 901 may be reflected offthe slab of material 150, and the ray 900 and the ray 901 may bereceived by the probe 602(1). In some embodiments, the 602(1) may send asignal representing the received ray 900 and the received ray 901 and/orthe ray 900 and the ray 901 to an optical metrology unit, for example,the optical metrology unit 127 and/or computer 124 of FIG. 15 A, foranalysis and determination of properties of the slab of material 150.

In some embodiments, the slab of material 150 may include multiplelayers. For example, the slab of material 150 may include a passivationlayer 151 made up of any suitable material, for example a syntheticpolymer, for example, polymide. In some embodiments, the slab ofmaterial 150 may include a metallic layer 152, for example, a metalizedpattern layer. The metallic layer 152 may be made up of any suitablemetal, for example, copper. In some embodiments, the slab of material150 ay include a silicon layer 153. The silicon layer 153 may be made upof silicon or silicon dioxide.

In some embodiments, semiconductor wafers are covered with a passivationlayer. Typical thicknesses of polymide passivation layers range fromfive micrometers (μm) to twenty μm. In some embodiments, passivationlayers are thinned. In some embodiments, passivation layers have a totalthickness variation on the order of one μm to five μm.

In some embodiments, the probe 602(1) may irradiate the slab of material150 with the ray 900 and the ray 901. There may be no difference betweenthe ray 900 and the ray 901 as the rays 900 and 901 leave the probe602(1). The ray 900 may be reflected by the air-passivation layerinterface. The ray 901 may be reflected by the passivation layer-metalinterface. The ray 900 and the ray 901 may have different propertieswhen they are received by the probe 602(1). For example, spectralanalysis of the 900 and the 901 may reveal a difference in the distancestraveled by the ray 900 and the ray 901 through an interferometricanalysis. Differences between the ray 900 and the ray 901 may includesuch things as a difference in wavelength, a difference in energy forexample, due to diffraction, and/or a delay due to a travel time. Forexample, the ray 900 and the ray 901 may be indistinguishable whentransmitted, and they may be transmitted together. The ray 900 and theray 901 may be separately reflected with ray 900 being reflected by theair-passivation-layer interface and the ray 901 passing through thepassivation layer to be reflected by thepassivation-layer-metallic-layer interface.

In some embodiments, based on the differences between the ray 900 andthe ray 901, it may be possible to determine the thickness of thepassivation layer 151. For example, the probe 602(1) may send signalsregarding the ray 900 and the ray 901 to the optical metrology unit 127which may determine the thickness of the passivation layer 151.Additionally or alternatively, the probe 602(1) may transmit the ray 900and the ray 901 to the optical metrology unit 127, for example, via asingle-mode optical fiber 107. The optical metrology unit 127 mayperform the interferometric analysis and determine the distancestraveled by the ray 900 and the ray 901 and the differences between thedistances traveled by the ray 900 and the ray 901.

Modifications, additions, or omissions may be made to the system 1600without departing from the scope of the present disclosure. For examplethe wafer 150 may include any number of layers which may reflect anynumber of corresponding rays.

FIG. 16B illustrates a simulated measurement of a slab of material thatincludes multiple layers as obtained by an FD-OCT probe according to atleast one embodiment described in the present disclosure. For example,FIG. 16B may include inteferogram results based on radiation received bya probe, for example, probe 602(1) of FIG. 16A in response toirradiating a slab of material 150 with a ray 900 and a ray 901. In FIG.16B, 900 may indicate a first distance, for example a distance traveledby ray 900 as determined by an interferometric analysis. In FIG. 16B,901 may indicate a second distance, for example a distance traveled byray 901 as determined by an interferometric analysis. By observing thedifference between the two distances 900 and 901, it may be possible todetermine a thickness of the passivation layer 151.

FIG. 17A illustrates an arrangement an FD-OCT probe for measurement of aslab of material that includes multiple layers according to at least oneembodiment described in the present disclosure. The probes 602(2) may bethe same as or substantially similar to the probes 602 described abovewith regard to FIGS. 7-13 and 15-16.1 n general, the probe 602(2) mayirradiate the slab of material 150 with ray 1001, the ray 1001 may bereflected off the slab of material 150, and the ray 1001 may be receivedby the probe 602(2). In some embodiments, the probe 602(2) may send asignal representing the received ray 1001 to an optical metrology unit127 and/or computer 124 for analysis and determination of properties ofthe slab of material 150. Additionally or alternatively, the probe602(2) may transmit the ray 900 and the ray 901 to the optical metrologyunit 127, for example, via a single-mode optical fiber 109. The opticalmetrology unit 127 may perform the interferometric analysis anddetermine the distances traveled by the ray 900 and the ray 901 and thedifferences between the distances traveled.

In some embodiments, the slab of material 150 may be the same as orsubstantially similar to the slab of material 150 of FIG. 16A.

In some embodiments, the probe 602(2) may irradiate the slab of material150 with the ray 1001. The ray 1001 may be reflected by the air-siliconinterface. The ray 1001 may be spectrally analyzed to reveal a distancetraveled by the ray 1001 through an interferometric analysis. In someembodiments, substantially all, or most of the radiation reflected mayhave the same distance traveled. This may indicate that only one surfacereflected radiation.

Modifications, additions, or omissions may be made to the system 1700without departing from the scope of the present disclosure. For examplethe wafer 150 may include any number of layers.

FIG. 17B illustrates a simulated measurement of a slab of material thatincludes multiple layers as obtained by an FD-OCT probe according to atleast one embodiment described in the present disclosure. For example,FIG. 17B may include inteferogram results based on radiation received bya probe, for example, probe 602(2) of FIG. 17A in response toirradiating a slab of material 150 with a ray 1001. In FIG. 17B, 1000may indicate a distance, for example a distance traveled by ray 1001 asdetermined by an interferometric analysis. By observing that there isonly one peak in the interferogram, it may be possible to conclude thatonly one reflecting interface was reached by the radiation. This may beindicative of the absence of a passivation layer on this surface of theslab of material 150.

FIG. 1S. 18A-18B illustrate a flowchart of an example process 1800 ofmeasuring a topography of a slab of material according to at least oneembodiment described in the present disclosure. In some embodiments, oneor more of the operations associated with the process 1800 may beperformed by the systems 600-1100 and 1300 of FIGS. 6-11 and 13, thesystem 1550 of FIG. 15A, or the system 1560 of FIG. 15B. In someembodiments, one or more of the operations associated with the process1800 may be performed by any suitable system, apparatus, or device. Forexample, the computer 124 of FIGS. 1-3 5-13, and 15A-B, the opticalmetrology unit 127 of FIGS. 15A and 15B, and/or the computing system1500 of FIG. 22 may perform one or more of the operations associatedwith the process 1800. Although illustrated with discrete blocks, thesteps and operations associated with one or more of the blocks of theprocess 1800 may be divided into additional blocks, combined into fewerblocks, or eliminated, depending on the desired implementation.

With reference to FIG. 18A, at block 1805 a substantially-flat referencesurface may be positioned on a chuck. For example, a mirror may be usedas the substantially-flat reference surface. The wafer chuck 160 of FIG.15A or 15B may be an example of the chuck of the block 1805.

At block 1810 one or more elements within two or more frequency-domainoptical-coherence tomography (OCT) probes may be actuated. The actuationmay occur at the same time. The probes 602 of the FIGS. 6-11, 13, 15A,and 15B may be examples of the OCT probes of the block 1810. In someembodiments, the actuation may cause the OCT probes to perform one ormore operations, including operations associated with block 1815 andblock 1820.

At the block 1815 the substantially-flat reference surface may beirradiated at at-least one location by the two or more OCT probes. Rays900 and 901 of FIG. 16A may be examples of the irradiation of the block1815. The probes 602 of the FIGS. 6-11, 13, 15A, and 15B may be examplesof the OCT probes of the block 1815.

At the block 1820 radiation may be synchronously detected by the two ormore OCT probes. The radiation may have been reflected from thesubstantially-flat reference surface or transmitted through thesubstantially-flat reference surface. The returning rays 900 and 901 ofFIG. 16A may be examples of the radiation being synchronously detectedat the block 1820. The probes 602 of the FIGS. 6-11, 13, 15A, and 15Bmay be examples of the OCT probes of the block 1820.

At block 1825 at least an interference-pattern of the radiation detectedby each of the OCT probes may be spectrally analyzed to determine atopography of the substantially-flat reference surface. For example,after the radiation is detected by the OCT probes at the block 1820, asignal indicative of the detected radiation, or the detected radiationitself, may be transmitted to an optical metrology unit for analysis.For example, the probes 602 of the FIGS. 6-11, 13, 15A, and 15B may beexamples of the OCT probes of the block 1825. After the probes receivethe radiation, it may be transmitted over single-mode optical fibers,such as, for example, the single-mode optical fiber 107 and thesingle-mode optical fiber 109 of FIG. 15A to the optical metrology unit127 of FIG. 15A. The optical metrology unit 127 of FIG. 15A may performthe spectral analysis of the block 1825.

In some embodiments, the substantially-flat reference surface may bemoved relative to the OCT probes such that multiple locations of thesubstantially flat reference surface may be irradiated and thereflections therefrom may be spectrally analyzed. For example, the XYmotion stage 170 of FIG. 15A may be used to move the wafer chuck 160relative to the probes 602. For example, the XY motion stage 170 maycause the scan multiple locations of the substantially-flat referencesurface. Thus, the block 1825 may include determining a topography ofmultiple locations of the substantially-flat reference surface.

At block 1830 a slab of material may be positioned on the chuck. Forexample, the slab of material 150 of FIG. 15A or 15B may be the slab ofmaterial of the block 1830. The wafer chuck 160 of FIG. 15A or 15B maybe an example of the chuck of the block 1830.

With reference to FIG. 18B, at block 1835 the one or more elementswithin the two or more OCT probes may be actuated. The actuation mayoccur at the same time. The probes 602 of the FIGS. 6-11, 13, 15A, and15B may be examples of the OCT probes of the block 1835. In someembodiments, the actuation may cause the OCT probes to perform one ormore operations, including operations associated with block 1840 andblock 1845.

At the block 1840 the slab of material may be irradiated at at-least onelocation by the two or more OCT probes. Rays 900 and 901 of FIG. 16A maybe examples of the irradiation of the block 1840. The probes 602 of theFIGS. 6-11, 13, 15A, and 15B may be examples of the OCT probes of theblock 1840.

At the block 1845 radiation may be synchronously detected by the two ormore OCT probes. The radiation may have been reflected from the slab ofmaterial or transmitted through the slab of material. The returning rays900 and 901 of FIG. 16A may be examples of the radiation beingsynchronously detected at the block 1845. The probes 602 of the FIGS.6-11, 13, 15A, and 15B may be examples of the OCT probes of the block1845.

At block 1850 at least an interference-pattern of the radiation detectedby each of the OCT probes may be spectrally analyzed to determine atopography of the slab of material. For example, after the radiation isdetected by the OCT probes at the block 1845, a signal indicative of thedetected radiation, or the detected radiation itself, may be transmittedto an optical metrology unit for analysis. For example, the probes 602of the FIGS. 6-11, 13, 15A, and 15B may be examples of the OCT probes ofthe block 1850. After the probes receive the radiation, it may betransmitted over single-mode optical fibers, such as, for example, thesingle-mode optical fiber 107 and the single-mode optical fiber 109 ofFIG. 15A to the optical metrology unit 127 of FIG. 15A. The opticalmetrology unit 127 of FIG. 15A may perform the spectral analysis of theblock 1850.

In some embodiments, the slab of material may be moved relative to theOCT probes such that multiple locations of the slab of material may beirradiated and the reflections therefrom may be spectrally analyzed. Forexample, the XY motion stage 170 of FIG. 15A may be used to move thewafer chuck 160 relative to the probes 602. For example, the XY motionstage 170 may cause the scan multiple locations of the slab of material.Thus, the block 1850 may include determining a topography of multiplelocations of the slab of material.

In some embodiments, the multiple locations of the slab of material thatare irradiated may correspond to the multiple locations of thesubstantially-flat reference surface. For example, the XY motion stage170 may be set to position the wafer chuck 160 in the same positionswhile each of the slab of material and the substantially-flat referencesurface are irradiated and measurements are taken therefrom.Additionally or alternatively, the same scan pattern may be used to scanthe slab of material as is used to scan the substantially-flat referencesurface. For example, each of the slab of material and thesubstantially-flat reference surface may be scanned using the sameraster pattern.

At block 1855 the topography of the slab of material may be calibratedby adjusting the topography of the slab of material based on thetopography of substantially-flat reference surface. For example, thetopography of the slab of material may include an indication of a heightor thickness of the slab of material at multiple locations. Likewise,the topography of the substantially flat reference material may includean indication of a height or thickness of the substantially-flatreference surface at multiple locations. Additionally or alternatively,the topography of the substantially-flat reference surface may includevariations in the measurements of the surface of the substantially-flatreference surface. The multiple locations of the topography of the slabof material may correspond to the multiple locations of the topographyof the substantially flat reference surface. For example, with referenceto the system 1550 of FIG. 15A, the topography of the substantially-flatreference surface may reveal irregularities in the stand 101, the waferchuck 160, the XY motion stage 170, or other elements of the system1550. The topography of the slab of material 150 may be corrected basedon the topography of the substantially-flat reference surface. Forexample, the topography of the slab of material 150 may be normalizedbased on the topography of the substantially flat reference surface. Forexample, any irregularities in the topography of the substantially flatreference surface may be subtracted from, or added to, the topography ofthe slab of material 150.

Modifications, additions, or omissions may be made to the method 1800without departing from the scope of the present disclosure. Further, theorder of operations may vary according to different implementations. Forexample, the blocks 1830-1850 may occur first, followed by the blocks1805-1825.

FIG. 19 illustrates an example system 1900 for inspecting a slab ofmaterial 150, including multiple frequency domain optical coherencetomography (FD-OCT) probes 602 and a thin-film gauge 132 according to atleast one embodiment described in the present disclosure. Morespecifically, FIG. 19 describes multiple probes 602(1) . . . 602(n) foracquiring data synchronously for measurement of thickness and topographyof the slabs of material. The FD-OCT probes 602(1) . . . 602(n) withinthe system 1900 correspond to the probes 602 of FIGS. 6-11, 13, 15A-16A,and 17A. Similarly, the slab of material 150, including passivationlayer 151, metallic layer 152, and silicon layer 153, within the system1900 corresponds to the slab of material 150 of FIGS. 15A-16A and 17A.

In some embodiments, the system 1900 includes a thin-film gauge 132,which may be configured to measure a thickness of one or more layers ofthe slab of material 150. For example, the thin-film gauge 132 may beconfigured to measure a thickness of the passivation layer 151. Thethin-film gauge 132 may be configured to irradiate the slab of material150 and detect radiation reflected from the slab of material 150. Thethin-film gauge 132 may be configured to measure the thickness of theone or more layers of the slab of material 150 using, or in connectionwith the computing system 124. For example, the computing system 124 mayperform one or more calculations with relation to measurements taken by,or detections of the thin-film gauge 132. In some embodiments, thethin-film gauge 132 may include a spectrometer 122. In these or otherembodiments, the thin-film gauge 132 may be connected, to one of thespectrometers 122 of one of the probes 602. Any of these spectrometers122 may perform one or more calculations with relation to themeasurements taken by, or detections of the thin-film gauge 132.

In some embodiments, the thickness of a layer of the slab of material150 may be determined by irradiating the slab of material 150 withradiation from the thin-film gauge 132 and detecting radiation reflectedfrom the slab of material 150 at the thin-film gauge 132. The thin-filmgauge 132 may irradiate the slab of material 150 with visible orultraviolet radiation and detect reflected visible or ultravioletradiation. Suitable wavelengths of irradiation include wavelengths fromtwo hundred nanometers (nm) to eight hundred nm. The thin-film gauge132, or one of the spectrometer 122 of one of the probes 602, may beconfigured to spectrally analyze radiation reflected by the slab ofmaterial 150 to determine a thickness of the one or more layers of theslab of material 150.

In some embodiments, a model of film on a slab of material 150 may beused when determining the thickness of the one or more layers of theslab of material 150. For example, the detected radiation may becompared to results of calculations made based on the model. Forexample, an exact stacked layer model of thin film residing on ametallic substrate may be used in determining the thickness of the oneor more layers of the slab of material 150.

In some embodiments, in determining the thickness of the one or morelayers of the slab of material 150, dispersion of the radiation may beanalyzed and/or determined. For example, measured radiation may becompared with a dispersion model. For example, a complex refractiveindex of the material may be compared with measured reflected radiation.In determining the thickness of the one or more layers, a refractiveindex of 1.5 to 1.7 may be used.

In some embodiments, the thin-film gauge 132 may include any suitabledevice for measuring thickness of a material, such as, for example, anoptical reflectometer, an optical ellipsometer, or a spectralellipsometer. In some embodiments, the thin-film gauge 132 may beconfigured to measure radiation having a wavelength of two hundrednanometers or more. In some embodiments, the thin-film gauge 132 may beconfigured to irradiate the slab of material 150 with radiation thatpropagates collinearly with radiation emitted from the beam-formingassembly 118.

In some embodiments, the determined thickness of the slab of material150 may be adjusted by the thickness of the one or more layers. Forexample, the determined thickness of the slab of material 150 may beadjusted by subtracting the thickness of a top layer of the slab ofmaterial 150. For example, a slab of material 150 may include a siliconsubstrate layer 153, a metallic layer 152, and a passivation layer 151.The thin-film gauge 132 may be used to determine the thickness of thepassivation layer 151. The determined thickness of the slab of material150 may be adjusted by subtracting off the thickness of the passivationlayer 151.

In some embodiments the system 1900 may include an XY motion stage, astand, a wafer chuck, and pins similar to or the same as what wasillustrated in FIGS. 15A and 15B. As described above with regard to FIG.15A, the system 1900 may be configured to calculated a topography and/orthickness map of the slab of material, including a thickness of one ormore layers, at multiple locations.

FIG. 20 illustrates a flowchart of an example process 2000 of measuringproperties of a slab of material that includes multiple layers,according to at least one embodiment described in this disclosure. Insome embodiments, one or more of the operations associated with themethod 2000 may be performed by the system 1900 of FIG. 19. Blocks ofthe method 2000 may be performed by any suitable system, apparatus, ordevice. For example, the computing system 1500 of FIG. 22 may performone or more of the operations associated with the method 2000. Althoughillustrated with discrete blocks, the steps and operations associatedwith one or more of the blocks of the method 2000 may be divided intoadditional blocks, combined into fewer blocks, or eliminated, dependingon the desired implementation.

At block 2010, a slab of material including a passivation layer may bepositioned relative to a frequency-domain optical-coherence tomography(OCT) probe. For example, the slab of material 150 of FIG. 19 may bepositioned relative to one or more of the probes 602 of FIG. 19. Theslab of material may be positioned a known distance from the probes. Forexample, the back surface of the slab of material 150 may be positioneda known distance from the probe 602(1).

At block 2030, the slab of material may be irradiated by the OCT probeat a location. For example, the probe 602(1) of FIG. 19 may irradiatethe slab of material 150 of FIG. 19.

At block 2040 first radiation reflected from the slab of material may bedetect by the OCT probe. For example, the probe 602(1) of FIG. 19 maydetect radiation reflected from the slab of material 150. The firstradiation may be reflected from one or more surfaces of the slab ofmaterial 150. For example, the slab of material 150 may include multiplelayers, including a passivation layer, a metallic layer, and a siliconlayer. The first radiation may include radiation reflected from anair-passivation-layer interface. The first radiation may also includeradiation reflected from the passivation-layer-metallic-layer interface.

At block 2050, at least an interference pattern of the first radiationdetected by the OCT probe may be analyzed to determine a thickness ofthe slab of material. For example a spectrometer and/or interferometermay be used to determine a change in the first radiation detected atblock 2040 from the radiation transmitted at the block 2030. Thedetermined thickness may correspond to one or more layers of the slab ofmaterial. For example, the determined thickness may correspond to all ofthe layers passivation layer 151, metallic layer 152, and silicon layer153 together.

At block 2060, the slab of material may be irradiated by a thin-filmgauge. For example, the thin-film gauge 132 of FIG. 19 may irradiate theslab of material 150 of FIG. 19. The slab of material may be irradiatedwith visible or ultraviolet light. The wavelength of the radiation usedto irradiate the slab of material may be based on a type of materialthat is assumed to be in the top layer of the slab of material.

At block 2070, second radiation reflected from the slab of material maybe detected by the thin-film gauge. For example, the thin-film gauge 132of FIG. 19 may detect radiation reflected from the slab of material 150of FIG. 19. The second radiation may be reflected from one or moresurfaces of the slab of material. For example, the slab of material mayinclude multiple layers, including a passivation layer, a metalliclayer, and a silicon layer. The second radiation may include radiationreflected from an air-passivation-layer interface. The second radiationmay also include radiation reflected from thepassivation-layer-metallic-layer interface.

At block 2080, the second radiation may be spectrally analyzed todetermine a thickness of the passivation layer. For example, thethin-film gauge 132 of FIG. 19 may analyze the radiation detected atblock 2070 and determine a thickness of the passivation layer 151 ofFIG. 19. For example a spectrometer may be used to determine a change inthe second radiation detected at block 2070 from the radiationtransmitted at the block 2060. As discussed above, the second radiationmay include radiation reflected from more than one layer of the slab ofmaterial. Differences in the reflections from the different layers maybe used to determine the thickness of one or more of the layers. Forexample, different wavelengths of radiation may have a peak in energyreflectance.

At block 2090, the determined thickness of the slab of material may beadjusted by subtracting the thickness of the passivation layer from thethickness of the slab of material. For example, the thickness determinedat block 2050 may include the passivation layer 151, the metallic layer152, and the silicon layer 153. At block 2090, the thickness of thepassivation layer 151, as determined at block 2080, may be subtractedfrom the thickness determined at the block 2050 resulting in adetermination of the thickness of the metallic layer 152 and the siliconlayer 153 without the passivation layer 151.

Modifications, additions, or omissions may be made to the method 2000without departing from the scope of the present disclosure. For example,in some embodiments, the blocks 2060, 2070, and 2080 may occur before,or at the same time as the blocks 2030, 2040, and 2050. Further, theorder of operations may vary according to different implementations.

FIG. 21 illustrates a flowchart of another example process 2100 ofmeasuring properties of a slab of material that includes multiplelayers, according to at least one embodiment described in thisdisclosure. In some embodiments, one or more of the operationsassociated with the method 2100 may be performed by the system 100 ofFIG. 1 or the system 1900 of FIG. 19. Blocks of the method 2100 may beperformed by any suitable system, apparatus, or device. For example, thecomputing system 1500 of FIG. 22 may perform one or more of theoperations associated with the method 2100. Although illustrated withdiscrete blocks, the steps and operations associated with one or more ofthe blocks of the method 2100 may be divided into additional blocks,combined into fewer blocks, or eliminated, depending on the desiredimplementation.

At block 2110, a slab of material including a passivation layer may bepositioned relative two or more frequency-domain optical-coherencetomography (OCT) probes. For example, the slab of material 150 of FIG.19 may be positioned relative to two or more of the probes 602 of FIG.19, for example, the probe 602(1) of FIG. 19 and the probe 602(n) ofFIG. 19. The slab of material may be positioned a known distance fromeach of the probes. For example, the back surface of the slab ofmaterial 150 may be positioned a known distance from the probe 602(1)and from the probe 602(n).

At block 2120 one or more elements within each of the two or more OCTprobes may be actuated at a same time to cause an operation through thetwo or more OCT probes. The operations may include irradiating the slabof material and detecting radiation from the slab of material. Forexample, one or more elements within each of the probes 602(1) and602(n) of FIG. 19 may be actuated at the same time to cause both of theprobes 602(1) and 602(n) to irradiate the slab of material 150 of FIG.19 at a first same time. Further, both of the probes 602(1) and 602(n)may detect radiation at a second same time.

At block 2123 the slab of material may be irradiated at a location bytwo or more OCT probes. In some embodiments, block 2123 may be part ofblock 2120 such that the slab of material is irradiated by the two ormore OCT probes at the same time.

At block 2124 first radiation reflected from the slab of material and/orsecond radiation transmitted through the material may be synchronouslydetected by two or more OCT probes. In some embodiments, block 2124 maybe part of block 2120 such that the first radiation and/or the secondradiation is detected by the two or more OCT probes at the same time.The first radiation may be reflected from one or more surfaces of theslab of material 150. For example, the slab of material 150 may includemultiple layers, including a passivation layer, a metallic layer, and asilicon layer. The first radiation may include radiation reflected froman air-passivation-layer interface. The first radiation may also includeradiation reflected from the passivation-layer-metallic-layer interface.

At block 2150 at least an interference pattern of the first radiationand/or the second radiation detected by each of the OCT probes may bespectrally analyzed to determine a thickness of the slab of material.For example an interferometric analysis may be used to determine atravel time for radiation between when it was transmitted at block 2123and when it was detected at block 2124. The travel time may be convertedinto a distance between the probe and a surface of the slab of material.After determining a distance between each of the probes, and opposingsides of the slab of material, a thickness of the slab of material maybe determined based on the distances between each of the probes and theopposing sides of the slab of material and the distance between theprobes.

At block 2160, the slab of material may be irradiated by a thin-filmgauge. For example, the thin-film gauge 132 of FIG. 19 may irradiate theslab of material 150 of FIG. 19. The slab of material may be irradiatedwith visible or ultraviolet light. The wavelength of the radiation usedto irradiate the slab of material may be based on a type of materialthat is assumed to be in the top layer of the slab of material.

At block 2170, third radiation reflected from the slab of material maybe detected by the thin-film gauge. For example, the thin-film gauge 132of FIG. 19 may detect radiation reflected from the slab of material 150of FIG. 19. The third radiation may be reflected from one or moresurfaces of the slab of material. For example, the slab of material mayinclude multiple layers, including a passivation layer, a metalliclayer, and a silicon layer. The third radiation may include radiationreflected from an air-passivation-layer interface. The third radiationmay also include radiation reflected from thepassivation-layer-metallic-layer interface.

At block 2180, the third radiation may be spectrally analyzed todetermine a thickness of the passivation layer. For example, thethin-film gauge 132 of FIG. 19 may analyze the radiation detected atblock 2170 and determine a thickness of the passivation layer 151 ofFIG. 19. For example a spectrometer may be used to determine a change inthe third radiation detected at block 2170 from the radiationtransmitted at the block 2160. As discussed above, the third radiationmay include radiation reflected from more than one layer of the slab ofmaterial. Differences in the reflections from the different layers maybe used to determine the thickness of one or more of the layers. Forexample, different wavelengths of radiation may have a peak in energyreflectance.

At block 2190, the determined thickness of the slab of material may beadjusted by subtracting the thickness of the passivation layer from thethickness of the slab of material. For example, the thickness determinedat block 2150 may include the passivation layer 151, the metallic layer152, and the silicon layer 153. At block 2190, the thickness of thepassivation layer 151, as determined at block 2180, may be subtractedfrom the thickness determined at the block 2150 resulting in adetermination of the thickness of the metallic layer 152 and the siliconlayer 153 without the passivation layer 151.

Modifications, additions, or omissions may be made to the method 2100without departing from the scope of the present disclosure. For example,in some embodiments, the blocks 2160, 2170, and 2180 may occur before,or at the same time as the blocks 2120 and 2150. Further, the order ofoperations may vary according to different implementations.

FIG. 22 illustrates yet another example implementation in accordancewith the embodiment of the present disclosure by depicting a computingdevice configured to facilitate measurement of properties of a slab ofmaterial through synchronized FD-OCT probes. More specifically, thepresent figure illustrates an example hardware configuration of thecomputer system 124 as a computing system 1500. The computer system 1500can include a set of instructions that can be executed to cause thecomputer system 1500 to perform any one or more of the methodsdisclosed. The computer system 1500 may operate as a standalone deviceor may be connected, e.g., using a network, to other computer systems orperipheral devices.

In a networked deployment, the computer system 1500 may operate in thecapacity of a server or as a client user computer in a server-clientuser network environment, or as a peer computer system in a peer-to-peer(or distributed) network environment. The computer system 1500 can alsobe implemented as or incorporated across various devices, such as apersonal computer (PC), a tablet PC, a personal digital assistant (PDA),a mobile device, a palmtop computer, a laptop computer, a desktopcomputer, or any other machine capable of executing a set ofinstructions (sequential or otherwise) that specify actions to be takenby that machine. In an example implementation, the computer system 1500may be a mobile computing cum display device capable of being used by auser. Further, while a single computer system 1500 is illustrated, theterm “system” shall also be taken to include any collection of systemsor sub-systems that individually or jointly execute a set, ormultiple-sets, of instructions to perform one or more computerfunctions.

The computer system 1500 may include a processor 1502 e.g., a centralprocessing unit (CPU), a graphics processing unit (GPU), or both. Theprocessor 1502 may be a component in a variety of systems. For example,the processor 1502 may be part of a standard personal computer or aworkstation. The processor 1502 may be one or more general processors,digital signal processors, application specific integrated circuits,field programmable gate arrays, servers, networks, digital circuits,analog circuits, combinations thereof, or other now known or laterdeveloped devices for analyzing and processing data The processor 1502may implement a software program, such as code generated manually (i.e.,programmed).

The computer system 1500 may include a memory 1504, such as a memory1504 that can communicate via a bus 1508. The memory 1504 may include,but is not limited to computer readable storage media such as varioustypes of volatile and non-volatile storage media, including but notlimited to random access memory, read-only memory, programmableread-only memory, electrically programmable read-only memory,electrically erasable read-only memory, flash memory, magnetic tape ordisk, optical media and the like. In one example, the memory 1504includes a cache or random access memory for the processor 1502. Inalternative examples, the memory 1504 is separate from the processor1502, such as a cache memory of a processor, the system memory, or othermemory. The memory 1504 may be an external storage device or databasefor storing data. The memory 1504 is operable to store instructionsexecutable by the processor 1502. The functions, acts or tasksillustrated in the figures or described may be performed by theprogrammed processor 1502 executing the instructions stored in thememory 1504. The functions, acts or tasks are independent of theparticular type of instructions set, storage media, processor orprocessing strategy and may be performed by software, hardware,integrated circuits, firm-ware, micro-code and the like, operating aloneor in combination. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing and the like.

As illustrated, the computer system 1500 may or may not further includea touch-sensitive display unit 1510, for outputting determinedinformation as well as receiving a user's touch-gesture based inputs,such as drag and drop, single tap, multiple-taps, etc. The display 1510may act as an interface for the user to see the functioning of theprocessor 1502, or specifically as an interface with the software storedin the memory 1504 or in the drive unit 1506.

Additionally, the computer system 1500 may include an input device 1512configured to allow a user to interact with any of the components ofsystem 1500. The computer system 1500 may also include a disk or opticaldrive unit 1506. The disk drive unit 1506 may include acomputer-readable medium 1518 in which one or more sets of instructions1514, e.g. software, can be embedded. Further, the instructions 1514 mayembody one or more of the methods or logic as described. In a particularexample, the instructions 1514 may reside completely, or at leastpartially, within the memory 1504 or within the processor 1502 duringexecution by the computer system 1500.

The present disclosure contemplates a computer-readable medium thatincludes instructions 1514 or receives and executes instructions 1514responsive to a propagated signal so that a device connected to anetwork 1526 can communicate voice, video, audio, images or any otherdata over the network 1526. Further, the instructions 1514 may betransmitted or received over the network 1516 via a communication portor interface 1520 or using a bus 1508. The communication port orinterface 1520 may be a part of the processor 1502 or may be a separatecomponent. The communication port 1520 may be created in software or maybe a physical connection in hardware. The communication port 1520 may beconfigured to connect with a network 1516, external media, the display1510, or any other components in computing system 1500, or combinationsthereof. The connection with the network 1516 may be establishedwirelessly as discussed later. Likewise, the additional connections withother components of the system 1500 may be established wirelessly. Thenetwork 1516 may alternatively be directly connected to the bus 1508.

The network 1516 may include wireless networks, Ethernet AVB networks,or combinations thereof. The wireless network may be a cellulartelephone network, an 802.11, 802.16, 802.20, 802.1Q or WiMax network.Further, the network 1516 may be a public network, such as the Internet,a private network, such as an intranet, or combinations thereof, and mayutilize a variety of networking protocols now available or laterdeveloped including, but not limited to TCP/IP based networkingprotocols. The system is not limited to operation with any particularstandards and protocols. For example, standards for Internet and otherpacket switched network transmission (e.g., TCP/IP, UDP/IP, HTML, HTTP)may be used.

The present subject matter comprising multiple FD-OCT probes forcharacterization of one or more properties of a slab of material,at-least enables a substantially high data-acquisition speed throughsynchronized FD-OCT probes, i.e. through measuring the distances betweenthe probes and slab at the same instant of time. This in turnsuccessfully overcomes the influence of vibration of the slab ofmaterial during the process of slab-characterization, which may includethe determination of one or more properties of the slab of material asdiscussed above. Moreover, the present subject matter provides a systemlow on manufacturing costs through employment of a centralizedspectrometer-detector combination.

While specific language has been used to describe the disclosure, anylimitations arising on account of the same are not intended. As would beapparent to a person in the art, various working modifications may bemade to the method in order to implement the inventive concept as taughtherein.

The drawings and the forgoing description give examples of embodiments.Those skilled in the art will appreciate that one or more of thedescribed elements may well be combined into a single functionalelement. Alternatively, certain elements may be split into multiplefunctional elements. Elements from one embodiment may be added toanother embodiment. For example, orders of processes described hereinmay be changed and are not limited to the manner described herein.

Moreover, the actions of any flow diagram need not be implemented in theorder shown; nor do all of the acts necessarily need to be performed.Also, those acts that are not dependent on other acts may be performedin parallel with the other acts. The scope of embodiments is by no meanslimited by these specific examples. Numerous variations, whetherexplicitly given in the specification or not, such as differences instructure, dimension, and use of material, are possible. The scope ofembodiments is at least as broad as given by the following claims.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any component(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or component of any or all the claims.

We claim:
 1. An apparatus for inspecting a slab of material, the slab ofmaterial including a passivation layer, the apparatus comprising: afrequency-domain optical-coherence tomography (OCT) probe configured to:irradiate the slab of material at a location; and detect radiationreflected from the slab of material; a spectral-analysis moduleconfigured to analyze an interference pattern of the radiation detectedthe OCT probe to determine a thickness of the slab of material at thelocation; and a thin-film gauge configured to determine a thickness ofthe passivation layer of the slab of material at the location; such thatthe determined thickness of the slab of material at the location may beadjusted based on the thickness of the passivation layer.
 2. Theapparatus of claim 1, wherein the thin-film gauge comprises an opticalreflectometer.
 3. The apparatus of claim 2, wherein the opticalreflectometer is configured to measure radiation having a wavelength of200 nanometers or more.
 4. The apparatus of claim 2, wherein the opticalreflectometer is configured to irradiate the slab of material withradiation that propagates collinearly with radiation emitted by the OCTprobe.
 5. The apparatus of claim 1, wherein the thin-film gaugecomprises an optical ellipsometer.
 6. The apparatus of claim 1, whereinthe thin-film gauge comprises a spectral ellipsometer.
 7. The apparatusof claim 1, wherein the OCT probe comprises a first OCT probe, theapparatus further comprising: a second OCT probe; and a centralizedactuation mechanism connected to the first OCT probe and the second OCTprobe and configured to actuate one or more elements in each of thefirst OCT probe and the second OCT probe at a same time duringirradiation of the slab of material to cause a synchronized detection ofthe radiation from the slab of material by the first OCT probe and thesecond OCT probe.
 8. The apparatus of claim 7, wherein at least two ofthe OCT probes are positioned on opposite sides of the slab of material.9. The apparatus of claim 7, wherein the thin-film gauge comprises areflectometer.
 10. A method comprising: positioning a slab of materialincluding a passivation layer relative a frequency-domainoptical-coherence tomography (OCT) probe; irradiating the slab ofmaterial at a location by the OCT probe; detecting, by the OCT probe,first radiation reflected from the slab of material; spectrallyanalyzing at least an interference pattern of the first radiationdetected by the OCT probe to determine a thickness of the slab ofmaterial; irradiating, by a thin-film gauge, the slab of material;detecting, by the thin-film gauge, second radiation reflected from theslab of material; spectrally analyzing the second radiation to determinea thickness of the passivation layer; and adjusting the determinedthickness of the slab of material by subtracting the thickness of thepassivation layer from the thickness of the slab of material.
 11. Amethod comprising: positioning a slab of material including apassivation layer relative to two or more frequency-domainoptical-coherence tomography (OCT) probes; actuating one or moreelements within each of the two or more OCT probes at a same time tocause an operation through the two or more OCT probes, the operationcomprising: irradiating the slab of material at a location by the two ormore OCT probes; and synchronously detecting, by the two or more OCTprobes, first radiation reflected from the slab of material and/orsecond radiation transmitted through the slab of material; spectrallyanalyzing at least an interference pattern of the first radiation and/orthe second radiation detected by each of the OCT probes to determine athickness of the slab of material; irradiating, by a thin-film gauge,the slab of material; detecting, by the thin-film gauge, third radiationreflected from the slab of material; spectrally analyzing the thirdradiation to determine a thickness of the passivation layer; andadjusting the determined thickness of the slab of material bysubtracting the thickness of the passivation layer from the thickness ofthe slab of material.